MOSCOW (Reuters) - Kremlin critic Alexei Navalny said on Tuesday he had lost 80 percent of sight in his right eye after an assailant threw green liquid in his face last week, but could not get treatment abroad because of a travel ban.

Navalny, who hopes to run for the presidency next year, organized the biggest anti-government protests in years in March, and has become the most prominent opposition challenger to President Vladimir Putin, who is widely expected to run for what would be a fourth presidential term in March 2018.

A 40-year-old former lawyer who has made a name for himself inside Russia investigating the finances of top government officials, Navalny was jailed for 15 days for his part in the March protests. He is calling for more on June 12.

Opinion polls suggest he does not pose a serious threat to Putin at the ballot box, but his muckraking investigations have angered the Kremlin. Prime Minister Dmitry Medvedev, the target of one, earlier this month dismissed Navalny as one of several "political conmen."

On Thursday, a man whom Navalny's supporters have identified as a member of a radical ultra-nationalist group threw green liquid in his face in Moscow. Footage of the attack, the second time Navalny had suffered such an assault in as many months, was shown on a pro-government TV channel.

At first, it didn't seem serious since the green solution, known as "zelyonka", is sold as an antiseptic in Russian pharmacies, and while it stings the eyes is not harmful.

But Navalny said in a statement on Tuesday that he had suffered a chemical burn to his right eye as a result of the assault which had robbed him of 80 percent of his sight in that eye and that a doctor had told him she was sure the antiseptic must have been mixed with something else.

"For now the loss of sight is not irreversible," said Navalny. "I'm being actively treated and there's hope I will be healed." But he said he was unable to get treatment in a specialized clinic in Switzerland or Spain because of a foreign travel ban imposed over what he says is a politically-motivated embezzlement conviction.

The authorities have so far not opened an investigation into the attack on him, something Navalny said was the subject of an official complaint. He has accused the Kremlin of being linked to the attack. The Kremlin denies involvement in any political violence.

An opinion poll published by the Levada Center on Tuesday, based on the views of 1,600 people across the country showed 48 percent would vote for Putin if the election was held now against just 1 percent who said they would vote for Navalny.